Sea Level at −175 m off the Great Barrier Reef 13,600 to 17,000 Year Ago

Abstract

THE shelf around Australia, like many other continental shelves¹, has its edge chiefly^2–5 at depths of 120 to 130 m and is marked locally by terraces and notches, which register low stands of the sea during the Quaternary. From radiometric dates of shallow water fossils recovered from terraces in different parts of the world the lowest eustatic level of the sea during the past 35,000 yr is estimated to be −130 m, 16,000 yr ago⁶. More recent information, principally from Australian waters, suggests that the sea level may have stood much lower during this period. Dill⁷ and Conolly⁴ found submerged terraces with shallow water fossils and sediments between depths of 175 to 238 m in 39 of 78 narrow beamed echo sounder profiles made all around Australia. The continuity and consistent depth of the terraces indicate that there has been little tectonic warping of the continental margin since the terraces were formed. Similar features are reported off southern and Baja California⁷, and a shallow water mollusc from a deep terrace off Baja California has a radiocarbon date of 14,380±190 yr BP. Another record of shallow water fossils of similar age and depth relates to the south-eastern Caribbean Sea⁸, where fragments of algae and hermatypical corals at a depth of 157 m (Station index 1,203) have radiocarbon dates of 13,590±270 and 13,800±330 yr BP, and algae at 187 m (Station index 1,202) a date of 14,220±350 yr BP. These organisms did not necessarily live at sea level⁶ but, being reef dwellers, probably indicate shallow water. To date, the evidence from Australia is only morphological. Here we describe the occurrence and radiometric dates of two shallow water specimens collected in situ from deep terraces off the Great Barrier Reef during descents by J. J. V. in the Japanese research submersible Yomiuri in February 1969.